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Green tides originate from the rapid growth of green macroalgae and their large accumulation. In the past few decades, the severity and frequency of green tides have increased and the range of their geographical distribution has widened. In recent years, <i>Ulva meridionalis</i> Horimoto et Shimada has been reported in many countries. This species has stable morphological characteristics, and its length can reach 3 m in indoor cultures. Its cells contain pyrenoids, and the sporangium and gametangium of each cell contain 8 spores and 16 gametes, respectively, which confer a high proliferation potential. The phylogenetic tree constructed in this study showed that the Internal Transcribed Spacer sequence identified <i>U. meridionalis</i> with a high identification reliability, and the genetic relationship between <i>U. meridionalis</i> and <i>Ulva pertusa</i> in the ITS sequence was close. The haplotype network analysis clarified the relationship of the <i>U. meridionalis</i> samples collected from four different sea areas in China and indicated that they were closely related. Five haplotypes were identified: Hap_2 and Hap_1 were the most frequent, and they were also the haplotypes shared among the three groups. The degree of subspecies formation was not reached among these <i>U. meridionalis</i> samples collected from the Chinese seas. Up to 20 years ago, <i>U. meridionalis</i> had only been recorded in Japan. After 2011, it has been found to be widely distributed in the United States, China, French New Caledonia, French Polynesia, and Australia, where it proliferates. It has spread as a new kind of green tide-forming macroalga. The present study found that <i>U. meridionalis</i> is widely distributed in the Chinese seas; specifically, there have been small-scale blooms in the Bohai Sea, the Yellow Sea, and the South China Sea. Further investigations should focus on establishing whether <i>U. meridionalis</i> will cause large-scale green tide events in the future.